Vacuum insulating piping is employed for transport of cryogenic fluids within industrial facilities such as air separation plants. Typically, vacuum insulated piping consists of a concentric arrangement of outer and inner pipes with vacuum insulation provided between the pipes. The outer pipe serves as a casing to contain the vacuum insulation and the inner pipe while the inner pipe serves to conduct the cryogenic fluids.
The outer casing also serves to contain a vacuum within the insulation to allow the insulation to properly function. Typically, this vacuum is ten microns of mercury or less, and preferably 1 micron of mercury or less. Since piping sections can only be practically shipped at between about 20 and 30 feet, the piping sections are fabricated in a first location such as a factory where vacuum is applied to the insulation space between the outer casing and the inner pipe. The piping sections are then joined in the field with the use of couplings between the piping sections.
U.S. Pat. No. 4,515,397 discloses a vacuum insulated conduit having inner and outer pipes and insulation between the pipes in which the vacuum is applied to each section during the fabrication. End sections of the inner pipe project from opposite ends of the piping sections to allow the piping sections to be joined at their inner pipes in the field. Insulated couplings around the juncture of the pipes and a bellows-like arrangement provided at the ends of the pipes inhibit heat leakage at the junctions.
U.S. Pat. No. 6,695,358 discloses a male-female bayonet or pipe spool connector that is used in connection with the coupling of vacuum insulated piping sections. Each of the piping sections has a pump out valve to allow evacuation of each of the sections and ports with rupture disks to prevent destruction of the piping section upon failure of the inner pipe.
U.S. Pat. No. 6,216,745 again utilizes prefabricated piping sections to which a vacuum has been applied in which the outer casing or pipe is connected to the inner pipe by way of a bellows. Each section can be evacuated from a cap port in the field. Additionally, since the piping sections are joined, a clam-shell arrangement may be secured together in the field and then filled with an insulation. A similar arrangement is disclosed in U.S. Pat. No. 6,257,282 in which vacuum-like sleeves are applied to the juncture between pipes.
U.S. Patent Application 2006/0054234 discloses a cryogenic piping system that is formed within an insulation jacket that is a continuous conduit to form a single insulation space about an inner pipe along the length of the inner pipe which is also continuous. The insulation volume is evacuated by a vacuum pump to a moderate vacuum and thereafter is filled with a high purity condensable gas, for example carbon dioxide. Thereafter, the insulation volume is evacuated to a moderate vacuum. During operation of the piping system any remaining condensable gas is cooled and condensed by flowing cryogenic liquid within the pipe volume to act to further lower the pressure within the piping system.
The disadvantage of all of the prior art patents in which the separate piping sections are separately evacuated is that the valves and ports provided for such purposes present individual sites at which failure can occur. Moreover, separately forming the piping sections in a manner in which each of the sections is to maintain a vacuum adds to the fabrication costs of the sections. Additionally, complex connections between the sections such as separate insulated couplings and bayonet arrangements also add to the fabrication costs. With respect to U.S. Patent Application 2006/005423, although it is not discussed in this patent that the same could be made by forming the piping system in separate sections and fabricating the system in the field, nevertheless if this were somehow done, there would be no provision for ensuring that the connections between the sections would be free of defects from which the maintenance of vacuum would be lost.
As will be discussed, the present invention discloses a method of assembly vacuum insulated piping that overcomes many of the disadvantages of the prior art by allowing sections to be connected in an inexpensive and highly simplified manner as compared to the prior art, that does not have multiple service points for applying vacuum and yet allows the connections between sections to be tested for integrity during the field installation of the completed piping run.